1. Field of the Invention
The present invention relates to an optical apparatus for capturing confocal images and a method of three-dimensional measurement using the apparatus.
2. Discussion of the Background
By utilizing a confocal optical system, the position (hereinafter referred to as the height) of the object measured in the direction of the optical axis (hereinafter referred to as the z-axis direction) can be measured accurately. Before description of the prior art, the principle of measurement of the height by a confocal optical system is explained. A basic configuration of the confocal optical system is shown in FIG. 10. Illumination emitted from the point light source 101 passes through the half-mirror 102 and is refracted by the objective lens 103 to converge on the object. The light that is reflected by the object and enters the objective lens 103 again is converged by the objective lens 103 and then diverted by the half-mirror 102 toward the pinhole 104 disposed at the same position optically as the point light source 101. The amount of light that passes through the pinhole 104 is detected by the photodetector 105. This is the basic configuration of the confocal optical system. By using this optical system, the height of a point on the surface of the object can be measured in the following manner. When a point on the surface on which light for illumination is shone is at the position conjugate to the point light source 101, the light reflected from the point focuses on the position of the pinhole 104, another conjugate position. Therefore, a large amount of light passes through the pinhole 104. The amount of light passing through the pinhole 104 sharply decreases with the distance from the position conjugate to the point light source to the point on the surface. This makes it possible to calculate the height of the point by moving the object with respect to the object lens 103 (hereinafter referred to as Z scan) and finding the position where the output of the photodetector 105 becomes greatest. This is the principle of measurement of height by a confocal optical system.
Since a confocal optical system of the above basic configuration can measure only one point on the surface of the object, scanning in the X and Y directions is required for three-dimensional measurement. However, a primitive method of three-dimensional measurement whereby the object is moved in the X and Y directions in a systematic pattern with respect to the objective lens at each height of stepwise scanning in the Z direction takes a very long time. Therefore, another method that executes Z scanning stepwise and performs X-Y scanning quickly by means of a laser beam or a rotating disk called a Nipkow disk while stopping Z scanning (keeping the distance between the object and objective lens fixed) at each of consecutive heights is commonly used. More specifically, this method performs image acquisition by repeating the steps of capturing a confocal image and moving the object to the next position in the Z direction. For every pixel position it finds the confocal image (Z position) in which the intensity of the pixel at that pixel position is greatest by comparing the intensities of the pixels at that pixel position for all the confocal images acquired. Finally, it calculates the three-dimensional shape of the object.
The confocal image acquisition system is most important for this measurement. Therefore, the confocal image acquisition system is described below as the prior art. There are two types of confocal image acquisition systems: the X-Y scanning type such as the laser scanning type, Nipkow disk scanning type, and table scanning type and the nonscanning type having a plurality of confocal optical systems arrayed in parallel in the X and Y directions. According to the number of illumination spots projected onto the surface of the object, the confocal image acquisition systems can be divided into two types: single spot and multispot (multibeam). The former includes the laser scanning type and the table scanning type, and the latter, the Nipkow disk scanning type and the nonscanning type. Since the present invention relates to the latter multibeam type, the principle of this type of confocal image acquisition system is described below.
FIG. 11(a) shows the configuration of the Nipkow disk scanning confocal image acquisition system. FIG. 11(b) shows the pinholes of a Nipkow disk. The Nipkow disk 111 is disposed at a focal plane (image-forming plane) of the objective lens 8 consisting of lenses 8a and 8b and a diaphragm 9. The Nipkow disk 111 is rotated by the motor 112. The Nipkow disk 111 has pinholes arranged in a spiral. The images of the pinholes are moved over the surface of the object in a raster pattern by one rotation of the Nipkow disk 111. The Nipkow disk 111 was originally invented for raster scanning for television.
The Nipkow disk 111 is illuminated from above by the illuminating arrangement consisting of the light source 1, pinhole 2, and collimator lens 4. Illumination that passes through the pinholes of the Nipkow disk 111 is refracted by the objective lens 8 and forms the images of the pinholes (illumination spots) on the surface of the object A. The light that is reflected from each spot and passes through the objective lens 8 is refracted to converge on the corresponding pinhole. The amount of reflected light that passes through the pinhole is greatest when the surface reflecting the light is at the position conjugate to the pinhole and decreases abruptly with the distance from the conjugate position to the surface. This produces the effect of a confocal optical system. The light that passes through the pinholes is diverted by the beam splitter 113, passes through the image re-forming lens 114, and forms an image of the pinholes on the detector array 11. By rotating the Nipkow disk 111, the light reflected from the illumination spots, which sweep the surface of the object in a raster pattern, moves over the whole detector array 111 and thereby a confocal image is obtained. This confocal image acquisition system using a Nipkow disk is herein called prior art A.
Next, the nonscanning confocal image acquisition system is described below. The nonscanning confocal image acquisition system is not so common, but it is disclosed in Japanese patent application laid-open 265918/1992, 181023/1995, and 257440/1997 and described in a paper written by H. J. Tiziani, et al., xe2x80x9cThree-Dimensional Analysis by a Microlens-Array Confocal Arrangementxe2x80x9d, Applied Optics, Vol. 33, No. 4, pp. 567-572 (1994). The apparatus disclosed in Japanese patent application laid-open 265918/1992 and the one disclosed in Japanese patent application laid-open 257440/1997, which was invented by the same inventor as the present invention, are described below as examples of the nonscanning confocal image acquisition system.
First, the apparatus disclosed in Japanese patent application laid-open 265918/1992 is described with reference to FIG. 12. Illumination emitted from the light source 1 is refracted by the collimator lens 4 into parallel-ray light and shone over the pinhole array 7. The pinhole array 7 consists of a plurality of pinholes arranged on the same plane. Each pinhole of the pinhole array 7 performs the same function as a point light source, and the pinhole array 7 is equivalent to arrayed point light sources. The light passing through the pinholes of the pinhole array 7 passes through the half-mirror 121. The light is then converged by the objective lens 8 consisting of lenses 8a and 8b and a telecentric diaphragm 9 and shone on the object A in small spots. The light that is reflected from each spot on the object A enters the objective lens 8 converges on the corresponding pinhole of the pinhole array 7. The light is then diverted by the half-mirror 121 away from the pinhole array 7 to the detector pinhole array 10 aligned with the pinhole array 7 so that its pinholes are at the same position optically as the corresponding pinholes of pinhole array 7. The light that passes through pinholes of the pinhole array 10 is detected by the detector array 11 whose element detectors are disposed right behind the corresponding pinholes. The above described configuration is equivalent to a plurality of confocal optical systems disposed in parallel. Although the detector pinhole array 10 is made dispensable by using a CCD sensor with a low aperture ratio (the ratio of its sensor element to the pixel area) as shown in the publication, the more common configuration described above is herein referred to as prior art B.
Next, the apparatus disclosed in Japanese patent application laid-open 257440/1997 is described with reference to FIG. 13. Illumination generated by the light source 1 is emitted through the pinhole 2 functioning as a point light source. The light emitted from the pinhole 2 is refracted by the collimator lens 4 into parallel-ray light. The optical path branching optical element 131 is a polarizing beam splitter which linearly polarizes the illumination light passing through it. The illumination passing through the optical path branching optical element 131 is shone over the microlens array 132 and converged by individual lenses to their focal points. The pinhole array 7 is disposed in the focal plane of the microlens array 132 and is aligned with the microlens array 132 so that each pinhole is coaxial with and positioned at the focal point of the corresponding microlens of the microlens array 132. Therefore, the illumination that enters each microlens converges on the pinhole under it and passes through the pinhole. The light that passes through each pinhole of the pinhole array 7 enters the objective lens 8; the light is converged by the objective lens 8 and circularly polarized by the xc2xc-wavelength phase shifting plate 133 placed in the objective lens 8. Thus an image of the pinhole array 7 is formed on the object A. The objective lens 8 is a bidirectional telecentric lens consisting of lenses 8a and 8b and a telecentric diaphragm 9 constructed so that the magnification (ratio of the size of the image of the object A or pinhole array 7 to the object A or pinhole array 7) does not change if the object A or pinhole array 7 is moved along the optical axis.
The light that is reflected from each illumination spot on the object A and enters the objective lens 8 is polarized again by the xc2xc-wavelength phase shifting plate 133 into linearly-polarized light at right angles with the illuminating light and converges on the corresponding pinhole of the pinhole array 7. The reflected light that passes through each pinhole of the pinhole array 7 is refracted by the corresponding microlens of the microlens array 132 to become parallel-ray light. The light then enters the optical path branching optical element 131 and is diverted to a image re-forming optic system 134 because of its polarization perpendicular to the illuminating light. The diverted light enters the image reforming optic system 134 and converges to form an image of the microlens array 132 on the detector array 11. The image formed on the detector array 11 is a confocal image and converted into electrical signals. This apparatus is referred to as prior art C.
The major difference between prior art C and prior art B is that prior art C has only one pinhole array serving both as the illuminating pinhole array and the detecting pinhole array instead of separate two pinhole arrays. From this point of view, the apparatus disclosed in patent application 181023/1995 and that described in the paper of H. J. Tiziani, et al., xe2x80x9cThree-Dimensional Analysis by a Microlens-Array Confocal Arrangementxe2x80x9d, Applied Optics, Vol. 33, No. 4, pp. 567-572 (1994) may be included in prior art C.
Detecting a plurality of points simultaneously by using a two-dimensional detector is common in the prior art described above. However, there is a problem with three-dimensional measurement by these confocal image acquisition systems; if there are high- and low-reflectance regions or regular- and scatter-reflection regions together in the measuring field, it is difficult to measure both regions at the same time.
In three-dimensional measurement by a confocal optical system, the peak of the intensity of reflected light must be sought. If the peak intensity is greater than the saturation intensity of the detector, the peak intensity cannot be known. If the peak intensity is smaller than the noise, it also cannot be known.
If the brightness of the measuring field of measurement is uniform, the intensity of reflected light can be adjusted so the peak intensity does not exceed the saturation intensity of the detector and is sufficiently greater than the noise by appropriately determining the intensity of the illumination or the shutter speed (exposure). When there are regions of considerably different reflection intensities in the measuring field as described above, the peak intensities of the light reflected from high-reflectance parts exceed the saturation intensity of the detector if the intensity of the illumination or the shutter speed is determined so that the peak intensities of the light reflected from low-reflectance area are sufficiently greater than the noise, or the peak intensities of the light reflected from low-reflectance area become smaller than the noise if the intensity of the illumination or the shutter speed is determined so that the peak intensities of the light reflected from high-reflectance parts do not exceed the saturation intensity of the detector.
The ratio of a high reflection intensity to a low reflection intensity can be the fifth power of 10 or greater in an extreme case. On the other hand, the dynamic range of a CCD, a commonly used two-dimensional detector, is at most the third power of 10. Therefore, the above described problem can inevitably occur.
Coexistence of area with a very large difference between their intensities of reflection in a measuring field can cause not only the problem of the dynamic range but also another problem. Since a multispot confocal optical system shines a plurality of illumination spots onto the object, the output of the detector indicating the intensity of reflection from an illuminating spot can be affected by light reflected from other spots. This is because a beam that is reflected from a surface not at the focused position converges off the corresponding pinhole as the beam 3 in FIG. 14 and part of the beam enters adjacent pinholes.
When the reflectances of adjacent area of the surface are nearly equal, no problem occurs if unfocused light reflected from one part enters the pinhole corresponding to the other part, because the intensity of the light from the one part is far smaller than the intensity of reflection from the other part when the other part is at the focused position.
However, if there is a great difference between the reflectances of the points of the surface illuminated by adjacent spots, a problem occurs. That is, if unfocused light reflected from the high reflectance point enters the pinhole corresponding to the low reflectance point, the intensity of the light from the high reflectance point is greater than the intensity of the light from the low reflectance point even when the point is at the focused position. Therefore, the output of the detector indicating the intensity of reflection from the low reflectance point attains a peak at a position other than the focused position of the point, causing an incorrect measurement.
There are other problems to be solved with confocal image acquisition systems, not only a multibeam confocal image acquisition system. One of the problems is unevenness caused by various factors of the optical system. If a high-precision mirror without flaw and dirt is measured by a multibeam confocal image acquisition system, for example, the outputs of all elements of the detector must be equal when the mirror is placed at the focused position for each illumination spot (the focused position at which the object is brought into focus is slightly different for each spot because of the distortion of the objective lens""s image plane) but not equal in practice.
This problem can be caused by the nonuniformity in illumination when a laser is used for the illuminating light source. Since a laser beam has usually an intensity distribution of a Gaussian distribution, the intensities of illumination at the central part (central part of image) and peripheral part (peripheral parts of image) of the beam are different when simply expanding the laser beam to illuminate the pinhole array.
Another cause is the difference in the size of the pinholes. There may be chips and burs in the edges of pinholes occurring in the manufacturing process. Further, in a system having a microlens array as prior art C, the manufacturing quality of the microlens array has an effect on the unevenness. The sensitivities of the elements of the detector are not the same. Manufacturing errors in the objective lens and differences in the image angles of light rays can cause nonuniformity.
Accordingly, an object of the present invention is to solve the problems described above.
To attain this object, the present invention adds to a confocal image acquisition system a light intensity control means for adjusting the illumination light intensity by spots or slits corresponding to each element of the detector for performing simultaneous parallel confocal detection.
Instead of a light intensity control means for adjusting the illumination intensity, a light intensity control means for adjusting the intensity of light entering each element of the detector may be added to a confocal image acquisition system.
It is preferable to use a light intensity control portion of a liquid crystal panel (hereinafter referred to as a liquid crystal panel) for the light intensity control means.
By adjusting the intensity of illumination or the intensity of light entering the detector in pixels so that the peak of the intensity of reflected light for each pixel does not exceed the saturation intensity of the detector and is sufficiently greater than the noise, it becomes possible to accurately measure even an object having both high reflectance and low reflectance area. The nonuniformity of the optical system can also be corrected at the same time.
Further, the measurable range of intensity of reflection is substantially increased without using a detector having a wide dynamic range and increasing the number of bits for measurement for the wider dynamic range. Therefore, the computational quantity does not increase.
It is further preferable that the confocal image acquisition system of the present invention be provided with a focused position shifting means for bringing different parts of the object into focus in order along the optical axis and a processing and control means for processing the images obtained and adjusting the light intensity control means.
Confocal image acquisition systems having the novel configuration described above are designated as active confocal image acquisition apparatus.
By thus configuring a system, a confocal image acquisition system can be used as a general-purpose three-dimensional measuring system, and even more accurate measurement is made possible by the following methods.
A method of measuring the three-dimensional shape of an object using the active multibeam confocal image acquisition apparatus comprises the following steps:
setting the intensity of light entering all elements of the detector to a low intensity;
capturing a plurality of confocal images bringing different parts of the object into focus by the focused position shifting means;
composing an extended focus image by collecting for all image pixels the greatest value among the values of pixels at the same pixel position of the confocal images obtained;
computing the adjustment information for adjusting the elements of the light intensity control means so as to make the intensities of light entering all elements of the detector as close to a predetermined level as possible using the values of the pixels of the extended focus image;
adjusting the elements of said light intensity control means according to the adjustment information; and
performing three-dimensional measurement by capturing a plurality of confocal images bringing different parts of the object into focus by the focused position shifting means.
The extended focus image may also be produced by computing the true peak value for each pixel from the confocal images obtained by the second step of the above method by interpolation.
Another method comprises the following steps:
setting the intensity of light entering all elements of the detector to a high intensity;
capturing a blurred confocal image at a Z position at which all pixels are evidently out of focus;
determining the adjustment information for adjusting the elements of the light intensity control means so as to make the intensities of light entering all elements of the detector as close to each other as possible using the values of the pixels of the blurred confocal image;
adjusting the elements of the light intensity control means according to the adjustment information; and
performing three-dimensional measurement by capturing a plurality of confocal images bringing different parts of the object into focus by the focused-position shifting means. Other objects, features and advantages of the invention will hereinafter become more readily apparent from the following description.